The present invention relates to a coaxial transmission line or cable capable of radiating as well as transmitting high frequency electromagnetic energy.
Cables radiating high frequency are beneficially employed as a distributed source or receiver of signals wherever communications in the radio bandwidth are inhibited by structural obstructions. Common installation sites therefore include within or around buildings, garages, tunnels, as well as in areas where communications are otherwise unobstructed but where precisely controlled signal levels must be distributed over a distance without interfering with other nearby signals.
In its simplest form, a coaxial cable is comprised of an inner conductor, an outer conductor concentrically arranged about the inner conductor, and a dielectric layer interposed between the two conductors. In a non-radiating coaxial cable, the outer conductor is of sufficient thickness and conductivity to attenuate the normally incident electric field, thereby permitting the transmission of a signal with a minimum of signal ingress or egress.
To the extent that signal leakage through the outer conductor can not be totally eliminated, all coaxial transmission lines are radiating to some extent. In radiating coaxial cables, however, the coaxial cable acts as an antenna and radiates a portion of the transmitted signal over its entire length or over a defined part of the cable. These radiated signals are useful for transmitting radio frequency signals to, for example, a mobile receiver.
The signal level found at a point external to and at a specific distance from the radiating cable should be at a predictable ratio with the level maintained within the cable. This ratio is known as the coupling loss and is usually expressed in logarithmic scale (dB). Because the coupling phenomenon results from the voltage level found in the cable coupling to an external potential, the line attenuation of the radiating cable will vary depending on the environment of installation and the weather conditions associated therewith. This is particularly true where the cable is affixed directly to the ground or is in contact with other lossy planes.
Although signal leakage is required for the radiating cable to function, it remains necessary that the cable retain most of its signal transmission characteristics. It has been observed that in order to obtain the desired radiation intensity, the apertures in the outer conductor must be very large. The effect of large apertures, however, is to increase the resistance per axial length of the cable. Correspondingly, the attenuation (measured in Db/100 ft) of the internal TEM signal is also increased. It is well known that such elevated levels of attenuation place severe limitations on the distance that unamplified signals can be transmitted along the cable.
The provision of apertures in the outer conductor affects the mechanical properties of the cable as well. Compared to a solid metal sheath, the apertured conductor is less resistant to kinking and crushing during handling and installation of the cable. Further, the ability to withstand environmental conditions, specifically moisture ingress into the dielectric core, is reduced. Each of these problems may lead to electrical degradation of the cable.
German printed application No. 2,022,990 discloses a high-frequency cable in which the outer conductor is constructed by winding a ribbon or a wire-like material around a continuous, cylindrical dielectric spacer, which in turn concentrically surrounds the central conductor. High frequency energy radiates through the resulting gaps or openings in the outer conductor. A jacket of conventional insulating material is placed over the outer conductor. This cable configuration, while relatively inexpensive to manufacture, is heavy and subject to immediate moisture ingress through the turns of the helical outer conductor when the outer jacket is damaged.
U.S. Pat. No. 4,129,841 discloses a radiating coaxial cable which in addition to a conventional central conductor, insulating spacer, and outer conductor, further includes a plurality of cylindrical radiating elements which are individually placed and distributed along the extension of the cable but in uniformly spaced apart relation to one another. A thin insulating envelope is provided between the radiating elements and the outer conductor. Although this arrangement allows for uniform distribution of the outer field over the entire extension of the cable, it is heavy, difficult to install, and relatively expensive to manufacture.
U.S. Pat. No. 4,339,733 discloses a radiating cable which includes a center conductor surrounded by a dielectric core and a plurality of radiating sheaths disposed along the length of the dielectric core so as to be coaxial with the central, longitudinal axis of the cable. In addition to decreasing attenuation, the provision of additional sheaths reduces moisture ingression due to the fact that the additional layers of radiating sheaths and dielectrics constitute additional barriers to water penetration. However, the formation and integration of plural sheaths into the cable design requires additional material and manufacturing steps, thus increasing both the weight of the cable and the costs of production.